Reciprocating pumps are extensively used on oil wells. They generally consist of a polish rod with cooperating valves, that serve to pump liquids to surface as the polish rod is raised and lowered in the well. There are various types of drives used to raise and lower the polish rod. Examples of linear actuators include hydraulic rams and lifting screws.
A problem common to linear actuators, such as hydraulic rams and lifting screws, is that the longer they extend, the more they are prone to deflection. This deflection can lead to jamming, bending, or fatigue failure. Persons skilled in the art presently avoid deflection problems by increasing the diameter of the linear actuator or limiting the length of the reciprocating stroke of the linear actuator.
U.S. Pat. No. 3,777,491 (Bender) and U.S. Pat. No. 4,512,149 (Weaver) disclose telescopic fluid cylinders with movable ram portions which carry cable guides. The combination of cables with the telescopic fluid cylinders unit results in the polish rod being lifted a distance which is a multiple of the distance of travel of the movable ram.